This invention relates to delivering sulfur to soil to support plant growth.
Sulfur, when oxidized to its sulfate form, is an essential nutrient for plant growth. To provide the sulfur in a form suitable for application to soil, the sulfur is typically bulk blended with granular fertilizers such as phosphates, nitrates, ureas, and/or potashes to form a physical blend. This blend is then applied to soil by various means such as broadcasting or banding to supply the soil with sulfur, as well as additional nutrients found in the granular fertilizers.
One problem with such blends is that they undergo size segregation during handling and transportation as the particles settle, resulting in smaller particles and dust concentrating near the bottom of the bulk blend. Consequently, sulfur is not uniformly distributed throughout the blend, resulting in uneven sulfur dosage when the blend is applied to soil. For example, some treated areas may receive too much sulfur, whereas others may receive too little sulfur. There is a need, therefore, for a vehicle that delivers sulfur uniformly to soil.
Sulfur has also been incorporated in fertilizer compositions for a different purpose. Specifically, sulfur has been used in the manufacture of slow release fertilizer compositions as a relatively thick outer coating or shell firmly anchored to the surface of fertilizer particles. In such compositions, the objective is to provide slow release of the underlying fertilizer to the soil, not the delivery of sulfur to the soil for subsequent oxidation and plant utilization.
The invention provides a vehicle for delivering sulfur to soil that avoids the problem of uneven sulfur distribution associated with bulk blended compositions. Specifically, the invention features a fertilizer composition having a plurality of particles in which the particles include a fertilizer portion and a plurality of discrete sulfur platelets embedded within the fertilizer portion.
As used herein, a xe2x80x9cdiscrete sulfur plateletxe2x80x9d refers to a thin, discontinuous, sulfur fragment that may be substantially planar or substantially curved. For example, the platelet may be in the form of a partial sphere. Such platelets are in contrast to the relatively thick sulfur coatings and shells found in slow release fertilizer compositions that substantially cover the surface of the underlying fertilizer portion.
Examples of suitable fertilizer portions include phosphates (e.g., monoammonium phosphate, diammonium phosphate, single superphosphate, triple superphosphate, etc.), nitrates, ureas, potashes, and combinations thereof. The particles may also include one or more sulfates (e.g., calcium sulfate, magnesium sulfate, ammonium sulfate, and combinations thereof), one or more micronutrients (e.g., zinc, manganese, iron, copper, molybdenum, boron, chloride, cobalt, sodium, and combinations thereof), and/or one or more secondary nutrients (e.g., calcium, magnesium, and combinations thereof). The total elemental sulfur content of the particles preferably is no greater than about 20% by weight, more preferably no greater than about 10% by weight, and even more preferably no greater than about 5% by weight.
Upon application to soil, the sulfur platelets are released to the soil, along with the components of the fertilizer portion, and are oxidized to the sulfate form by soil microbial activity. Oxidation is facilitated by forming the platelets such that they have thicknesses no greater than about 100 microns, preferably no greater than about 10 microns. Because each particle in the composition contains sulfur platelets, the problem of uneven sulfur application is avoided, even if the particles undergo settling. Moreover, by embedding the sulfur platelets in the fertilizer portion, rather than providing sulfur solely as a firmly anchored coating over the outermost surface of the fertilizer particles, the particles provide a convenient vehicle for delivering both the sulfur and fertilizer components to the soil simultaneously.
The fertilizer composition is prepared according to a process that includes spraying a plurality of fertilizer-containing particles first with elemental sulfur and then with a slurry that contains fertilizer, or a precursor thereof, and thereafter curing the resulting sulfur-containing particles. The curing process may take a number of forms depending upon the composition of the slurry. For example, in the case of slurries containing a monoammonium or diammonium phosphate fertilizer precursor, the curing step includes sparging the sulfur-containing particles with ammonia and thereafter drying the particles to form particles in which sulfur platelets are embedded within monoammonium or diammonium phosphate fertilizer. On the other hand, where the slurry contains single super phosphate, triple superphosphate, urea, or potash, the ammonia sparge is not necessary and the particles are cured simply by drying.
The fertilizer slurry typically is a molten composition. If the slurry were deposited first, followed by sulfur, or if sulfur and the slurry were deposited simultaneously, areas of sulfur deposited on top of the molten slurry would have a tendency to flake off the particle. Spraying the particles first with sulfur and then with the fertilizer slurry avoids this problem. The molten slurry is deposited over the sulfur platelets but more strongly adheres to portions of the underlying fertilizer particle, rather than the sulfur platelets. Thus, both the fertilizer and the sulfur platelets remain in place, yet can be released at the appropriate time into soil.